ProbeForWrite (
    __inout_bcount(Length) PVOID Address,
    __in SIZE_T Length,
    __in ULONG Alignment
    )

/*++

Routine Description:

    This function probes a structure for write accessibility and ensures
    correct alignment of the structure. If the structure is not accessible
    or has incorrect alignment, then an exception is raised.

Arguments:

    Address - Supplies a pointer to the structure to be probed.

    Length - Supplies the length of the structure.

    Alignment - Supplies the required alignment of the structure expressed
        as the number of bytes in the primitive datatype (e.g., 1 for char,
        2 for short, 4 for long, and 8 for quad).

Return Value:

    None.

--*/

{

    ULONG_PTR EndAddress;
    ULONG_PTR StartAddress;

#define PageSize PAGE_SIZE

    //
    // If the structure has zero length, then do not probe the structure for
    // write accessibility or alignment.
    //

    if (Length != 0) {

        //
        // If the structure is not properly aligned, then raise a data
        // misalignment exception.
        //

        ASSERT((Alignment == 1) || (Alignment == 2) ||
               (Alignment == 4) || (Alignment == 8) ||
               (Alignment == 16));  //如果对齐值为其中的一个，则不产生断言

        StartAddress = (ULONG_PTR)Address;
        if ((StartAddress & (Alignment - 1)) == 0) {  //判断StartAddress是否为0地址打头(根据字节对齐指定多少位0)

            //
            // Compute the ending address of the structure and probe for
            // write accessibility.
            //

            EndAddress = StartAddress + Length - 1; //获得缓冲区的最后一个字节地址
            if ((StartAddress <= EndAddress) &&
                (EndAddress < MM_USER_PROBE_ADDRESS)) { //如果是合法用户地址

                //
                // N.B. Only the contents of the buffer may be probed.
                //      Therefore the starting byte is probed for the
                //      first page, and then the first byte in the page
                //      for each succeeding page.
                //
                // If this is a Wow64 process, then the native page is 4K, which
                // could be smaller than the native page size/
                //

                EndAddress = (EndAddress & ~(PageSize - 1)) + PageSize; //这里开始就不懂了。PageSize = 0，(EndAddress & ~(0x1000 - 0x1)) -> (EndAddress & 0)   result = 0 + PageSize?EndAddress == 0x1000????。。。
                do {
                    *(volatile CHAR *)StartAddress = *(volatile CHAR *)StartAddress;
                    StartAddress = (StartAddress & ~(PageSize - 1)) + PageSize;
                } while (StartAddress != EndAddress); //这里为什么死都要让StratAddress == EndAddress，那么不就等于Length = NULL了么。这样缓冲区的长度传进来代表什么。

                return;

            } else {
                ExRaiseAccessViolation();
            }

        } else {
            ExRaiseDatatypeMisalignment();
        }
    }

    return;
}

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